The present invention relates to a roots type fluid machine.
A roots type fluid machine is known which includes a housing, a pair of rotary shafts, a pair of rotors and a rotor chamber. The housing has a suction port and a discharge port formed therein, and the paired rotary shafts are rotatably arranged in parallel to each other in the rotor chamber. The rotors respectively including lobe and valley portions are rotatably mounted on the respective rotary shafts and engaged with each other in the rotor chamber. Fluid chambers are formed between the rotors and the inner surface of the rotor chamber. During the rotation of the rotors, the fluid chamber firstly communicates with the suction port, then is closed from the suction and discharge ports, and communicates with the discharge port. The volume of the fluid chamber is gradually increased while the fluid chamber is in communication with the suction port, and gradually decreased while the fluid chamber is closed or in communication with the discharge port, thus performing a pumping operation. That is, fluid is flowed in through the suction port, then compressed and discharged out through the discharge port.
FIG. 13 shows a conventional roots type fluid machine. Referring to the drawing, a rotor chamber 73 has an inner peripheral surface whose transverse section is formed by connecting two circles 71, 72 centered on axes O1, O2, respectively, and the angle formed between a line L1 connecting the axes O1, O2 and a line L2 connecting the axis O1 and an intersecting point (cusp) S or D of the two circles 71, 72 is X degree.
As shown in FIG. 13, the rotors 98, 99 are plane symmetrical to each other and, therefore, only one of the rotors, i.e. the rotor 98, will be explained (the same is applicable to the rest of the description). The rotor 98 is defined by the axis O1 of the rotary shaft 91, a plurality of imaginary lines Li, curved outlines Le and outer surfaces F. The imaginary lines Li extend radially from the axis O1 toward the respective apex ends T of the rotor 98 and are spaced angularly at a substantially equal angle. The number of the imaginary lines Li equals to the number n of lobe portions or valley portions of the rotor 98. The curved outline Le connects the bottom end B of the valley portion 93 and the apex end T of the lobe portion 92. The outer surface F is formed by the outline Le rotated and moved in the direction of the axis O1 for a distance corresponding to the axial length of the rotor 98. If the outline Le of the rotor 98 is formed by an involute curve, the rotor 98 collides with the rotor 99 at the top end of the lobe portion of the rotor 99. In order to forestall such collision, the outline Le of the rotor 98 is formed with an undercut so as to reduce the dead volume formed in the roots type fluid machine. Thus, in a general conventional roots type fluid machine, the outline Le is formed by an involute curve and an envelope curve which is described by the path of the top end of the lobe portion of the mating rotor. The rotor of the conventional roots type fluid machine shown in FIG. 13 is of a six-lobe configuration in which the value of n is six and each number of the lobe and valley portions is six.
In the conventional roots type fluid machine wherein the shape of the lobe portion 92 of the rotor 98 is narrowed toward the apex end T thereof, the moment of inertia of the rotor 98 is relatively small and, therefore, the rotor 98 may be driven easily to rotate at a high speed. The space for the rotor 98 in the rotor chamber 73 may be reduced, so that the volume of the fluid chamber 96 may be increased and the displacement by the rotor 98 may be increased for a small size of the roots type fluid machine.
However, in this conventional roots type fluid machine shown in FIG. 13, a large dead volume 30 is formed between the rotors 98, 99, so that power loss due to fluid leakage is relatively large and the noise tends to be generated by reexpansion of fluid.
For this reason, a roots type fluid machine has been disclosed in Japanese Patent Application Publication No. 2007-162476 by the present applicant. As shown in FIG. 14, the rotor 88 of the roots type fluid machine disclosed in the above Publication is of two-lobe or three-lobe configuration in which the value of n is two or three and each number of the lobe and valley portions is two or three. In the roots type fluid machine of the above Publication, the outline Le of the rotor 88 is formed by an arc 81A, an involute curve 82A and an envelope curve 83.
As shown in FIG. 14, the arc 81A, which forms a part of a circle 81 having its center at Q1 located on an imaginary line Li passing through the apex end T of the lobe portion and a radius R, extends from the apex end T to a first transition point C1 between the arc 81A and the involute curve 82B of the outline Le. Reference symbol R1 indicates the distance between the axis O1 of the rotor 88 and the center Q1 of the circle 81. The involute curve 82A, which is based on the circle 82 having its center Q2 located at the axis O1 and a radius r, extends from the first transition point C1 to a second transition point C2 connected to the envelop curve 83 of the outline Le. The involute curve 82A is formed continuously with the arc 81A. The envelope curve 83 extends from the second transition point C2 to the bottom end B of the outline Le and along outside of a path of the arc 81A of the lobe portion of the mating rotor 89. The envelope curve 83 is formed continuously with the involute curve 82A. According to the roots type fluid machine disclosed in Japanese Patent Application Publication No. 2007-162476, power loss and noise development may be reduced and stable volumetric efficiency may be obtained.
Therefore, the present invention is directed to providing a roots type fluid machine according to which power loss and noise development may be further reduced and stable volumetric efficiency ηV and a reliable and excellent overall thermal efficiency ηtad may be achieved.